Activation Energy | A-Level Chemistry

Last updated: 17 August 2026

Diagnose → Learn → Master: Every study-note section includes 15 free MCQs — start with 5 Diagnostic MCQs to identify gaps in your understanding, then complete 10 Mastery MCQs near the end to check what you have learned.

Some chemical reactions are energetically favourable yet do not happen immediately. Hydrogen and oxygen can remain together without instantly forming water, while a fuel can sit in air until a spark or flame is supplied. Something must first allow the reacting particles to cross an energy barrier.

That barrier is described by the activation energy. It connects several important ideas in reaction kinetics: collision theory, energy profile diagrams, Boltzmann distributions, temperature and catalysis.

Once these ideas are linked together, explanations of reaction rate become much more logical. Instead of simply memorising that a higher temperature makes a reaction faster, you can explain exactly what happens to the reacting particles and why the number of successful collisions increases.

Diagnostic Quiz

Can You See the Energy Barrier?

Try these five questions before reading the chapter. They check whether you already understand the basic connection between collisions, energy and reaction rate.

5 quick questions
Focus on the core activation-energy ideas
Every answer option is explained

Which statement best describes activation energy?

The total energy released when products form

This describes an energy change associated with the reaction rather than the energy barrier that must first be overcome.

The minimum energy required for a reaction to occur

Correct. Reacting particles must possess enough energy to overcome the activation-energy barrier before reaction can occur.

The average kinetic energy of all the particles

Average kinetic energy depends mainly on temperature. Activation energy is an energy threshold associated with the reaction pathway.

The energy difference between reactants and products

The energy difference between reactants and products is related to ΔH. Activation energy is measured between the reactants and the transition state.

On an energy profile diagram, from where should the activation energy of the forward reaction be measured?

From zero on the vertical axis to the peak

Activation energy is an energy difference relative to the reactants. The bottom of the graph is not automatically the energy of the reactants.

From the products to the reactants

The difference between reactant and product energy levels is associated with ΔH, not the forward activation energy.

From the reactant energy level to the peak

Correct. The peak represents the transition state, so the energy difference between the reactants and the peak is Eₐ for the forward reaction.

From the products to the bottom of the graph

Neither of these positions defines the forward activation-energy barrier.

Two reactant particles collide with energy greater than Eₐ, but no reaction occurs. Which explanation is possible?

Particles with energy greater than Eₐ can never react

Particles with energy ≥ Eₐ have sufficient energy to overcome the activation barrier. Something else must explain the unsuccessful collision.

The particles collided with an unsuitable orientation

Correct. Sufficient energy is necessary, but the particles may also need to collide in an orientation that allows the required bonds to break and form.

Their kinetic energy must have been zero

This contradicts the information that their collision energy was greater than Eₐ.

The activation energy must always be greater than the energy of the particles

Activation energy is a fixed threshold for a specified pathway. Some particles can certainly possess energies above that threshold.

What happens when the temperature of a reaction mixture is increased without changing the reaction pathway?

The activation energy decreases

Temperature changes the distribution of particle energies. It does not lower Eₐ when the reaction pathway remains the same.

Every particle immediately has energy greater than Eₐ

There is still a distribution of molecular energies. Higher temperature does not give every particle the same high kinetic energy.

The number of particles in the sample must increase

Heating redistributes the energies of the existing particles. It does not necessarily change the number of particles present.

A greater fraction of particles possesses energy ≥ Eₐ

Correct. This increases the number of collisions that have sufficient energy to overcome the activation barrier.

Which statement correctly describes the action of a catalyst?

It provides an alternative reaction pathway with a lower activation energy

Correct. The lower energy barrier allows a greater fraction of the particles to react successfully at the same temperature.

It increases the kinetic energy of every reactant particle

A catalyst does not heat the reaction mixture. At the same temperature, the distribution of particle kinetic energies remains unchanged.

It increases the enthalpy of the products

A catalyst does not change the energy levels of the reactants or products. ΔH therefore remains unchanged.

It makes all collisions successful

A lower Eₐ increases the proportion of energetically successful collisions, but not every collision must have the correct energy and orientation.

Activation energy and successful collisions

According to collision theory, reactant particles must collide before a reaction can occur. However, not every collision leads to reaction.

During a successful collision, existing bonds may need to weaken or break while new bonds begin to form. Reaching the required high-energy arrangement therefore requires energy.

Activation energy, Eₐ = minimum energy required for a reaction to occur

If colliding particles do not possess enough energy to overcome this barrier, they separate again without forming products.

Collision	Energy of collision	Orientation	Outcome
Collision 1	Less than Eₐ	Suitable	No reaction because there is insufficient energy
Collision 2	At least Eₐ	Unsuitable	No reaction because the particles are incorrectly oriented
Collision 3	At least Eₐ	Suitable	A successful collision can occur

Having energy ≥ Eₐ is necessary, but it does not guarantee reaction.

The particles must also collide in an orientation that allows the necessary bond changes to occur. A collision with sufficient energy but an unsuitable orientation may still be unsuccessful.

Why is there an energy barrier?

Reactant molecules do not normally change directly from intact reactants into stable products. As they approach and interact, bonds may be stretched, electron distributions change and repulsive interactions must be overcome.

The reacting system must therefore pass through a less stable, higher-energy arrangement before the products can form. Energy must first be supplied to reach this arrangement.

Activation energy can therefore be thought of as an energy threshold. Particles below this threshold cannot reach the required transition state during a collision.

Activation energy on an energy profile diagram

An energy profile diagram shows how the energy of a reacting system changes as the reaction proceeds from reactants to products.

The vertical axis represents energy, or enthalpy when an enthalpy profile is being considered. The horizontal axis represents the progress of the reaction. It does not represent time.

Feature	Meaning
Reactant energy level	The energy of the reacting substances before reaction
Peak of the curve	The transition state, or activated complex
Product energy level	The energy of the substances formed
Activation energy, Eₐ	The energy difference between the reactants and the transition state
Enthalpy change, ΔH	The energy difference between the products and reactants

The transition state

The highest point on the energy profile represents the transition state, sometimes called the activated complex.

This is an extremely short-lived, high-energy arrangement of atoms in which the bonding is changing. It should not be thought of as an ordinary stable substance that can simply be isolated and stored.

Eₐ(forward) = energy of transition state − energy of reactants

Always measure Eₐ from the reactant energy level.

The activation-energy arrow should not start from the bottom of the vertical axis. It starts at the energy level of the reactants and extends to the peak of the energy barrier.

Activation energy is not the same as ΔH

Activation energy and enthalpy change describe different energy differences.

ΔH = energy of products − energy of reactants

In an exothermic reaction, the products are lower in energy than the reactants and ΔH is negative. In an endothermic reaction, the products are higher in energy than the reactants and ΔH is positive.

Both reactions can still require an activation energy before products can form.

Quantity	Measured between	What it describes
Eₐ	Reactants and transition state	The energy barrier that must be overcome
ΔH	Reactants and products	The overall enthalpy change of the reaction

Forward and reverse activation energies

The same energy profile can also be read in the reverse direction. For the reverse reaction, the original products become the reactants.

Eₐ(reverse) = energy of transition state − energy of products

The forward and reverse activation energies are therefore generally different when the reactants and products have different energies.

ΔH = Eₐ(forward) − Eₐ(reverse)

Do not confuse the height of the barrier with the overall energy change.

An exothermic reaction can still have a large activation energy. A negative ΔH means that the products are lower in energy than the reactants; it does not mean that the reaction can start without first overcoming an energy barrier.

Activation energy and the Boltzmann distribution

At a particular temperature, the particles in a sample do not all possess the same kinetic energy. Some have relatively little energy, while others have considerably more.

This spread of molecular energies can be represented using a Boltzmann distribution.

Feature of the graph	Meaning
x-axis	Kinetic energy of the particles
y-axis	Number, or fraction, of particles possessing a particular energy
Area under the curve	Total number of particles represented
Eₐ line	The minimum energy threshold required for reaction
Area to the right of Eₐ	Particles possessing energy ≥ Eₐ

The curve begins at the origin, rises to a maximum and then decreases gradually as kinetic energy increases. It has a long high-energy tail that approaches the horizontal axis.

What does the Eₐ line mean?

Imagine drawing a vertical line at the activation energy. Particles represented to the left of this line possess less energy than Eₐ. Particles represented to the right possess energy equal to or greater than Eₐ.

Area to the right of Eₐ ∝ fraction of particles with energy ≥ Eₐ

A larger fraction beyond Eₐ means that more collisions have enough energy to overcome the activation barrier.

The area beyond Eₐ represents energetically capable particles, not guaranteed reactions.

Particles with energy ≥ Eₐ possess sufficient energy for reaction, but a suitable collision orientation may still be required.

Why does the total area under the curve matter?

If two Boltzmann curves describe the same sample, the total area under each curve is the same because the same total number of particles is being represented.

Increasing temperature redistributes the particles among different kinetic energies. It does not create additional particles simply because the shape of the curve changes.

How increasing temperature increases reaction rate

Increasing temperature generally increases reaction rate. There are two related effects that help explain this.

Particles move faster and collide more frequently

At a higher temperature, particles have a higher average kinetic energy. They move faster, so collisions generally occur more frequently.

This increase in collision frequency contributes to the increase in reaction rate, but it is not usually the most important part of the explanation.

A much larger fraction of particles can overcome Eₐ

Increasing temperature changes the shape of the Boltzmann distribution. Compared with the distribution at a lower temperature:

the peak becomes lower;
the curve becomes broader;
the peak occurs at a higher kinetic energy; and
a greater fraction of particles is found in the high-energy region.

The activation energy has not changed. Instead, a much larger fraction of particles now lies beyond the same Eₐ threshold.

Higher temperature → larger fraction with energy ≥ Eₐ → more successful collisions per unit time

This increase in the fraction of particles capable of overcoming Eₐ explains why a relatively modest rise in temperature can cause a much larger increase in reaction rate.

Incorrect idea: Increasing temperature lowers the activation energy.

Correct reasoning: For the same reaction pathway, Eₐ remains unchanged. Increasing temperature changes the Boltzmann distribution so that a greater fraction of particles has energy ≥ Eₐ.

What happens to the rate constant, k?

For a specified reaction, increasing temperature increases the rate constant, k. At the higher temperature, a greater proportion of collisions can overcome the activation-energy barrier.

This differs from increasing reactant concentration. Increasing concentration may increase the reaction rate because collisions become more frequent, but at constant temperature it does not by itself change k for the same reaction and conditions.

How a catalyst lowers the activation energy

A catalyst increases reaction rate by providing an alternative reaction pathway with a lower activation energy.

Eₐ(catalysed pathway) < Eₐ(uncatalysed pathway)

Because the new barrier is lower, a greater fraction of the reacting particles possesses sufficient energy to overcome it at the same temperature.

How is a catalyst shown on an energy profile diagram?

The catalysed pathway has a lower maximum than the uncatalysed pathway. The energies of the reactants and products remain unchanged.

A catalyst therefore lowers Eₐ but does not change the overall enthalpy change, ΔH.

ΔH(catalysed) = ΔH(uncatalysed)

How is a catalyst shown using a Boltzmann distribution?

At the same temperature, adding a catalyst does not change the Boltzmann distribution curve. The distribution of particle kinetic energies remains the same.

Instead, the required activation energy becomes lower. The Eₐ line for the catalysed pathway therefore lies further to the left than the Eₐ line for the uncatalysed pathway.

A larger fraction of the existing distribution now lies to the right of the lower activation-energy threshold.

A catalyst changes the pathway, not the kinetic energies of the particles.

Increasing temperature changes the Boltzmann distribution. Adding a catalyst at constant temperature leaves that distribution unchanged but lowers the energy threshold that particles must overcome.

Feature	Increasing temperature	Adding a catalyst
Boltzmann distribution	Changes shape	Unchanged at the same temperature
Activation energy	Unchanged for the same pathway	Lower for the alternative pathway
Fraction with sufficient energy	Increases	Increases because the threshold is lower
Frequency of successful collisions	Increases	Increases
Reaction rate	Increases	Increases
ΔH	Unchanged for the reaction	Unchanged

Activation energy and the rate constant, k

The rate constant, k, appears in a reaction's rate equation and reflects the kinetics of the reaction under specified conditions.

Increasing temperature increases k because a greater fraction of particles can overcome the activation-energy barrier.

A suitable catalyst introduces a different reaction pathway with a lower Eₐ. Reaction through the catalysed pathway is therefore faster at the same temperature.

Change	Effect on reaction rate	Effect on k
Increase reactant concentration	Usually increases	Unchanged at constant temperature for the same reaction conditions
Increase gaseous reactant pressure	Usually increases	Unchanged at constant temperature for the same reaction conditions
Increase temperature	Increases	Increases
Add a suitable catalyst	Increases	The reaction proceeds through a different, lower-Eₐ kinetic pathway

Do not confuse factors that change rate with factors that change k.

Increasing concentration can increase reaction rate because more collisions occur per unit time. At constant temperature, however, concentration does not change the activation-energy barrier or k for the same reaction conditions.

Worked examples with activation energy

Example 1: reading an energy profile

An energy profile has the following energy levels:

reactants = 40 kJ mol⁻¹;
transition state = 125 kJ mol⁻¹; and
products = 10 kJ mol⁻¹.

First find the activation energy of the forward reaction:

Eₐ(forward) = 125 − 40 = 85 kJ mol⁻¹

Next find the enthalpy change:

ΔH = 10 − 40 = −30 kJ mol⁻¹

The negative value shows that the forward reaction is exothermic.

For the reverse reaction:

Eₐ(reverse) = 125 − 10 = 115 kJ mol⁻¹

Answer: Eₐ(forward) = 85 kJ mol⁻¹, ΔH = −30 kJ mol⁻¹ and Eₐ(reverse) = 115 kJ mol⁻¹.

Example 2: explaining the effect of temperature

Suppose the same reaction is carried out first at temperature T₁ and then at a higher temperature T₂.

A strong explanation should connect the Boltzmann distribution to successful collisions.

At the higher temperature, the Boltzmann distribution becomes broader and its peak becomes lower and occurs at a higher kinetic energy. A greater fraction of particles therefore possesses energy ≥ Eₐ. The particles also move faster, so collisions occur more frequently. Hence successful collisions occur more frequently and the reaction rate increases.

Notice that Eₐ has not decreased. The reaction pathway remains unchanged.

Example 3: explaining the effect of a catalyst

Suppose an uncatalysed reaction has an activation energy of 80 kJ mol⁻¹ and a catalyst provides an alternative pathway with an activation energy of 45 kJ mol⁻¹.

At the same temperature, the Boltzmann distribution itself is unchanged. However, many more particles possess energy of at least 45 kJ mol⁻¹ than possess energy of at least 80 kJ mol⁻¹.

The catalyst provides an alternative reaction pathway with a lower activation energy. A larger fraction of particles therefore possesses sufficient energy to overcome the activation barrier, increasing the frequency of successful collisions and hence the reaction rate.

Common mistakes with activation energy

Common mistake	Why it is wrong	Correct approach
Drawing the Eₐ arrow from the bottom of the energy diagram	Eₐ is an energy difference relative to the reactants	Draw the arrow from the reactant energy level to the peak
Treating Eₐ as the same quantity as ΔH	Eₐ measures the barrier, whereas ΔH compares products with reactants	Identify the reactants, transition state and products separately
Saying every collision with energy ≥ Eₐ produces products	Suitable collision orientation may also be required	State that sufficient energy and suitable orientation are required
Saying increasing temperature lowers Eₐ	Temperature changes particle energies, not the reaction pathway	State that the fraction of particles with energy ≥ Eₐ increases
Drawing a larger total area under the higher-temperature Boltzmann curve	The same sample still contains the same total number of particles	Keep the total areas under the two curves equal
Saying a catalyst shifts the Boltzmann curve to higher energy	A catalyst does not increase the temperature	Keep the curve unchanged and lower the Eₐ threshold
Saying a catalyst changes ΔH	The reactant and product energy levels remain unchanged	Show a lower pathway with the same starting and finishing energy levels
Saying increased concentration increases k	At constant temperature, concentration changes collision frequency rather than k	Distinguish an increase in reaction rate from an increase in the rate constant
Calling the horizontal axis of an energy profile “time”	The graph does not show how long the reaction takes	Label it reaction progress or reaction coordinate

Activation-energy checklist

Read an energy profile

Identify the reactant and product energy levels.
Locate the transition state at the peak.
Measure Eₐ from the reactants to the peak.
Measure ΔH from the reactants to the products.

Read a Boltzmann distribution

Use kinetic energy on the x-axis.
Interpret the area under the curve as the total number of particles.
Locate the activation-energy threshold.
Interpret the area beyond Eₐ as particles with energy ≥ Eₐ.

Explain temperature

State that average kinetic energy increases.
Describe how the Boltzmann distribution changes.
State that the fraction with energy ≥ Eₐ increases.
Conclude that successful collisions occur more frequently.

Explain a catalyst

State that an alternative pathway is provided.
State that the new pathway has a lower Eₐ.
Keep the Boltzmann curve unchanged at the same temperature.
Remember that ΔH is unchanged.

Mastery Quiz

Activation Energy Mastery Quiz

Now apply the ideas from the chapter. These questions test energy-profile calculations, Boltzmann distributions, temperature, catalysts and the distinction between reaction rate and rate constant.

10 application-focused questions
Energy diagrams and kinetic reasoning
Every answer option is explained

An energy profile has reactants at 35 kJ mol⁻¹, a transition state at 120 kJ mol⁻¹ and products at 15 kJ mol⁻¹. What is the activation energy of the forward reaction?

20 kJ mol⁻¹

20 kJ mol⁻¹ is the magnitude of the energy difference between the reactants and products, not the activation energy.

105 kJ mol⁻¹

120 − 15 gives the activation energy of the reverse reaction because the reverse reaction begins from the product energy level.

85 kJ mol⁻¹

Correct. Eₐ(forward) = 120 − 35 = 85 kJ mol⁻¹.

120 kJ mol⁻¹

The absolute energy assigned to the transition state is not itself Eₐ. Activation energy is an energy difference.

A reaction has Eₐ(forward) = 70 kJ mol⁻¹ and ΔH = −25 kJ mol⁻¹. What is Eₐ(reverse)?

45 kJ mol⁻¹

For an exothermic forward reaction, the products are lower in energy, so the reverse reaction must climb a larger barrier rather than a smaller one.

95 kJ mol⁻¹

Correct. ΔH = Eₐ(forward) − Eₐ(reverse), so −25 = 70 − Eₐ(reverse), giving Eₐ(reverse) = 95 kJ mol⁻¹.

70 kJ mol⁻¹

The forward and reverse barriers would be equal only if the reactants and products were at the same energy level.

25 kJ mol⁻¹

25 kJ mol⁻¹ is the magnitude of ΔH, not an activation energy.

A reaction is heated from T₁ to T₂, where T₂ > T₁. Which description of its Boltzmann distribution is correct?

The peak becomes lower, the distribution becomes broader and the fraction beyond Eₐ increases

Correct. Heating spreads the distribution towards higher kinetic energies, producing a much larger fraction of particles with energy ≥ Eₐ.

The peak becomes higher and Eₐ moves to the left

Heating does not change Eₐ for the same reaction pathway, and the higher temperature curve has a lower rather than higher peak.

The total area under the curve becomes larger because there are more particles

For the same sample, the total number of particles is unchanged. The total area under the distribution therefore remains the same.

Every particle moves to an energy greater than Eₐ

Even at the higher temperature there is a range of particle energies, with many particles still possessing energy below Eₐ.

Two samples of the same reacting mixture are at the same temperature. A catalyst is added to one sample. Which statement is correct?

The catalysed sample has a Boltzmann curve shifted towards higher energy

At the same temperature, the distribution of kinetic energies is unchanged. A catalyst does not heat the particles.

The catalysed sample contains more particles

Adding a catalyst does not increase the number of reactant particles being represented by the Boltzmann distribution.

The Boltzmann curve is unchanged, but the activation-energy threshold is lower

Correct. A catalyst provides a lower-Eₐ pathway, so a larger fraction of the same molecular-energy distribution can react.

The product energy level becomes lower

Catalysis does not alter the energies of the reactants or products, so ΔH remains unchanged.

Why can a small increase in temperature sometimes produce a much larger percentage increase in reaction rate?

The activation energy becomes almost zero

Temperature does not lower Eₐ for the same reaction pathway.

The number of reactant molecules increases greatly

Heating the sample does not automatically create more reactant molecules.

Collision frequency is the only factor that changes

Collision frequency increases, but the major kinetic effect is that a larger fraction of particles possesses sufficient energy to overcome Eₐ.

The fraction of particles lying in the high-energy tail beyond Eₐ can increase substantially

Correct. The Eₐ threshold lies in the high-energy part of the distribution, so changing the shape of the distribution can substantially increase the fraction beyond that threshold.

The concentration of a reactant is doubled while the temperature and all other conditions remain unchanged. Which statement is most appropriate?

Reaction rate may increase, but the rate constant k remains unchanged

Correct. More reactant particles per unit volume can increase collision frequency, but k does not change merely because concentration changes at the same temperature.

Both reaction rate and k must double

The effect on rate depends on the reaction order, and concentration does not by itself change k at constant temperature.

The activation energy becomes half as large

Concentration does not alter the reaction pathway or the activation-energy barrier.

The Boltzmann distribution shifts to higher kinetic energy

The distribution of kinetic energies is determined by temperature, which has not changed.

Reaction P has ΔH = −60 kJ mol⁻¹ and a high activation energy. Which conclusion is justified?

P must be extremely fast because ΔH is negative

A negative ΔH describes the overall thermodynamic energy change, not the speed at which the activation barrier is crossed.

P is exothermic but may still be slow because few particles overcome its large Eₐ

Correct. A reaction can release energy overall while still having a large kinetic barrier.

P cannot occur because an exothermic reaction cannot have an activation energy

Exothermic reactions commonly have activation-energy barriers.

The products must lie above the reactants on the energy profile

A negative ΔH means that the products lie below the reactants.

A catalyst lowers the forward activation energy of an exothermic reversible reaction. What happens to the reverse activation energy?

It remains unchanged because catalysts work only in the forward direction

A catalyst provides an alternative pathway between the same reactants and products, so the pathway can also be followed in reverse.

It increases by the same amount that the forward activation energy decreases

The catalysed pathway has a lower transition-state energy, so the barrier is lower from both sides rather than higher in the reverse direction.

It also decreases

Correct. Lowering the transition-state energy lowers the activation barrier for both the forward and reverse reactions.

It becomes equal to ΔH

Activation energy and enthalpy change remain different quantities.

An uncatalysed reaction has Eₐ = 100 kJ mol⁻¹. A catalyst provides a pathway with Eₐ = 55 kJ mol⁻¹. Why does the catalysed reaction proceed faster at the same temperature?

The catalyst raises the temperature of the reaction mixture

Catalysis does not require the temperature to increase.

A larger fraction of particles possesses energy ≥ 55 kJ mol⁻¹ than energy ≥ 100 kJ mol⁻¹

Correct. Lowering the threshold means a much larger fraction of the existing energy distribution can overcome the barrier.

The catalyst increases the total area under the Boltzmann curve

At the same temperature and for the same sample, the molecular-energy distribution does not acquire a larger total area simply because a catalyst is present.

The catalyst makes ΔH more negative

A catalyst does not change the reactant or product energy levels, so ΔH is unchanged.

A student compares two energy profiles for the same reaction. The catalysed profile has a lower peak, but both profiles begin and end at exactly the same energy levels. Which conclusion follows?

The catalyst changes ΔH but not Eₐ

The unchanged starting and finishing levels show that ΔH is unchanged, while the lower peak shows that Eₐ has decreased.

The catalyst changes neither Eₐ nor ΔH

The lower peak means that the activation-energy barrier has changed.

The catalyst makes the products less stable

The product energy level is unchanged, so the catalyst has not changed the relative energy of the products.

The catalyst lowers Eₐ while leaving ΔH unchanged

Correct. A catalyst changes the pathway and therefore the barrier, but not the initial or final energy levels of the reaction.

Frequently asked questions

Questions students often ask about activation energy

These questions connect activation energy with reaction profiles, molecular-energy distributions and the kinetic explanations used throughout A-Level Chemistry.

Can an exothermic reaction still have a large activation energy?

Yes. Exothermic means that the products are lower in energy than the reactants, so ΔH is negative. The reaction may still have to overcome a large activation-energy barrier before the products can form.

This explains why a reaction can be energetically favourable overall yet proceed very slowly at room temperature.

Does a larger activation energy always mean a slower reaction?

Under otherwise comparable conditions, a larger activation energy generally means that a smaller fraction of particles possesses sufficient energy to react, so the reaction tends to be slower.

Other kinetic factors can also affect reaction rate, so activation energies should be compared carefully when the reactions or conditions are very different.

Is the transition state the same as an intermediate?

No. A transition state corresponds to an energy maximum along a reaction pathway and is extremely short-lived.

A reaction intermediate, if present in a multi-step mechanism, corresponds to a local energy minimum between two transition states.

Why does increasing temperature not reduce activation energy?

Activation energy belongs to a particular reaction pathway. Increasing temperature changes the distribution of molecular kinetic energies, allowing more particles to overcome the existing barrier, but it does not create a different pathway.

Why does a catalyst not change ΔH?

ΔH depends only on the energy difference between the initial reactants and final products. A catalyst changes the route between these states but does not change their energy levels.

Does a catalyst lower the activation energy of the reverse reaction as well?

Yes. A catalyst provides an alternative pathway between the same reactants and products. The lower-energy pathway can be followed in either direction, so the activation barriers for both forward and reverse reactions are lowered.

Does a catalyst make the reacting particles move faster?

No. At the same temperature, the distribution of molecular kinetic energies remains unchanged.

The reaction becomes faster because the catalyst provides a pathway with a lower activation energy, not because the particles themselves move faster.

Why is the area beyond Eₐ so important when temperature increases?

Only particles with sufficient energy can overcome the activation barrier. Because Eₐ lies in the high-energy region of the distribution, increasing temperature can cause a substantial increase in the fraction of particles beyond that threshold.

The frequency of successful collisions therefore increases substantially.

Why is reaction progress used on an energy profile instead of time?

An energy profile describes the sequence of energetic and structural changes as reactants become products. The horizontal direction therefore represents progress along the reaction pathway rather than elapsed time.

A tall energy barrier does not mean that the graph is showing a longer period of time.

Does increasing concentration increase the rate constant?

No, not simply because the concentration has increased. At constant temperature for the same reaction conditions, increasing concentration can increase reaction rate by increasing the frequency of collisions.

The rate constant k is not increased merely by increasing reactant concentration.

Activation energy links collision theory to the energy changes that occur as a reaction proceeds. An energy profile shows the barrier that must be crossed, while a Boltzmann distribution shows how many particles possess enough energy to cross it. Increasing temperature raises the fraction of particles with energy ≥ Eₐ, whereas a catalyst provides a different pathway with a lower Eₐ. With these ideas secure, changes in reaction rate and rate constant can be explained in terms of what is actually happening to the reacting particles.